Refractories are used in numerous high-temperature, corrosive applications. One particularly difficult application is slagging coal gasifiers. Slagging coal gasifiers have highly-fluid and chemically-corrosive slags. These slags are among the most aggressive types encountered by refractories. The only category of refractories that are able to resist the highly corrosive conditions in slagging coal gasifiers is burned bricks that contain high amounts of chromic oxide. Typically, the burned-brick refractories used in slagging coal gasifiers are comprised of chromia-alumina, wherein the chromic oxide content may range from 20 to 100% by chemical analysis. The chromia-containing aggregate within these refractories may be sintered or fused, or combinations of sintered and fused materials. It is known to add fine chromic oxide to the matrix of the brick to improve slag resistance. U.S. Pat. No. 6,815,386 to Kwong et al. describes the addition of phosphates to burned brick with high chromia content to improve resistance to slag penetration.
High-chromia refractories are very expensive, and ways of improving the performance and useful life of these materials are being sought. Despite efforts to improve the performance of burned brick with high chromia contents, there is a continuing need for improvement, particularly with respect to slag penetration.
It is known that the addition of carbon to refractories in general limits slag penetration and improves corrosion resistance. In this respect, the non-wetting nature of carbon limits the penetration of liquid slag to a thin oxidized zone at the hot face of a refractory. This thin oxidized zone will vary in depth, but it is not unusual for the oxidized zone to be limited to several millimeters or less. (In contrast, carbon-free refractories typically exhibit much greater slag penetration, sometimes exceeding 75 millimeters).
However, the use of carbon in high-chromia brick such as chromia-alumina refractories used in slagging coal gasifiers has not heretofore been practiced. It has been believed by those skilled in the art that chromia, in the presence of carbon, will be reduced to chromium metal by the carbon at the high temperatures that are characteristic of the operation of slagging coal gasifiers. In this respect, temperatures can reach 1600° C. in slagging coal gasifiers. The reduction of chromia to chromium metal would be expected to have deleterious effects on the refractory lining.
The present invention overcomes these and other problems and provides a chromia-alumina brick containing carbon having improved resistance to slag penetration.